The central nervous system (CMS) poses a unique challenge to the immune system, as it is considered an immunologically specialized compartment that resides behind a non-fenestrated barrier and significantly limits the expression of antigen-presenting machinery. Because the CNS is compartmentalized and imposes strict regulatory mechanisms on the adaptive immune system, a multitude of pathogens that infect humans (e.g. HIV, herpes virus, measles virus, HTLV-1, JC virus, etc.) are capable of establishing persistence in the CNS. These persistent infections can give rise to severe behavioral alterations and neurological dysfunction, which adversely affects human health. Persistent infections that gain access to the CNS often do so by first establishing persistence in the periphery. This common scenario consequently presents biomedical researchers with the challenge of eradicating the pathogen from both the periphery as well as the CNS. Given that the CNS provides such a favorable environment to viruses seeking persistence, we postulate that achieving clearance from this specialized compartment will be far more challenging than from peripheral tissues. The proposed study will explore a murine model system in which an immunosuppressive virus is introduced through a peripheral route and then establishes prolonged persistence in the CNS despite the eventual clearance from the periphery. Clearance from the periphery coincides with a functional reactivation of virus-specific T cells (a scenario we hope to achieve in humans), yet this response is unable to prevent prolonged viral persistence within the CNS. Importantly, administration of exogenous memory T lymphocytes at the peak of CNS viral infection results in efficient clearance of the virus from both the periphery and the CNS. Thus, an adoptively transferred population of memory T lymphocytes can achieve successful clearance despite an inadequate endogenous immune response. The proposed study is designed to significantly advance our understanding of T cell immunity to persistent viral infections in the CNS by establishing the shortcomings of the endogenous T cell response (Specific Aim#1) and the modus operand of a highly successful adoptive response (Specific Aim#2). The main objective of this research is mechanistically define the factors that give rise to successful CNS viral clearance so that we are better able to supplement a failing (or inadequate) endogenous response.